The invention relates to a method for the comminution and particularly for the fine comminution of solid material with a rolling mill. More particularly, the invention relates to a method and mechanism for fluidizing a particulate material, directing the fluidized material under pressure into the roller gap so it is essentially squirted therein and further aiding the high velocity entering of the gap by entraining the material with a further flow of compressed air.
An economical comminution with the aid of rolling mills is possible only when the comminution rollers are driven with a sufficiently high circumferential speed. An essential problem exists in that the material to be comminuted should be supplied in a distributed manner uniformly over the entire width of the rollers so that the roller gap and the quantity charged can be related for optimum grinding effect. That is, the rollers must be positioned apart the proper distance relative to the particulate size of the material, and the amount of material introduced into the gap. This insures uniform treatment of the material across the length of the gap so that different effects are not attained along the gap and a compacting or compression and briquetting does not occur.
In accordance with the principles of the present invention, nonuniform comminuting and briquetting are avoided by introducing the charging material into the roller gap at a speed which is at least 80 per cent, and should be over 50 per cent of the rotational surface speed of the rolls. Generally, a material speed which is approximately equal to the circumferential speed of the grinding rolls is used. Also, the grinding rolls should have a circumferential speed of at least 4 m/sec. With the arrangement, the control of the charging quantity is obtained in a manner which prevents sloping out or nonuniform grinding along the width of the roller gap and also prevents conglomeration or briquetting or pressing of the material so that a uniform comminuted product results, rather than one which is lumpy and briquetted. Further, the charging material, in accordance with the present invention, is accelerated or propelled into the gap uniformly over the entire width of the roller gap in a manner wherein there is a homogeneous distribution of material over the entire roller length to obtain optimum grinding output. This presents advantages, particularly for the comminution of a charging material in the fine grain area, for example, with grain size below 1 min.
The acceleration or propelling of the charging material into the gap can be done by charging devices which operate mechanically, such as centrifugal devices. However, in a preferred embodiment of the invention, the charging material is accelerated and propelled by means of compressed air. An advantage of this lies in obtaining high acceleration of the material supplied for the roller gap in a simple manner whereby the air effectively supports the uniform distribution of the charging quantity over the entire length of the roller gap.
In accordance with the structure and method of the present invention, the circumferential speed of the grinding rollers should be at least 8 m/sec., and more advantageously, 10 m/sec. and higher with a range of over 8 m/sec. being preferred. Speed of introduction of material to the roller gap corresponds to the speed of the roller surface and is at least 80 per cent of the circumferential speed of the grinding rollers. Advantages of the present invention attain more economical grinding through the use of higher speed grinding rollers.
It is accordingly an object of the present invention to provide an improved comminuting device wherein a charging material is fluidized in the area of the feed to roller gap by means of compressed air. A further object is to provide a uniform and regulatable density of the charging material by such fluidizing so that optimum grinding output is attainable, and briquetting or compacting of portions of the material is prevented.
In accordance with the invention to increase the output of comminuted material, the charging material is preferably sifted before being supplied to the roller gap, and the fine portions which are sufficiently fine to be regarded as a finished product are withdrawn before passage through the rolling mill.
In accordance with the present invention, the structure has a rolling mill and a charging device wherein the roller gap is arranged horizontally and a delivery slot is positioned above the gap extending over the entire length thereof. The charging device is provided with means for acceleration of the charging material. In one form acceleration means are provided in the forms of nozzles positioned on at least one side of the material outlet slot of the charging device with the nozzles delivering a flow of compressed air to entrain the fluidized material and further deliver it to the gap at a velocity. With the aid of the nozzles a high acceleration of the charging material is obtained without wear on the parts and without the necessity of providing many mechanical parts or a complex apparatus for charging. Further, the jet effect of the individual nozzles aids in making the distribution of material more uniform across the entire area of the roller gap. In another form the material is accumulated in the charging device and is pressurized from above and is fluidized.
In a preferred embodiment, the comminution device in accordance with the present invention, the charging device in the area directly above the material outlet slot has mechanism for loosening and fluidizing the charging material. Such loosening devices may operate mechanically and have as an objective obtaining a uniform density of distribution for conduction into the slot leading to the roller gap. This enables uniform acceleration of the material for feeding into the roller gap and uniform distribution over the width of the gap. In a preferred arrangement, a pressure chamber is provided for the discharge device wherein its walls are porous and of elastic material with compressed air being delivered through the porous walls to fluidize the material in the chamber.
It is accordingly an object of the present invention to provide an improved comminuting device which comminutes material at a greater capacity and higher speed than heretofore possible, which attains comminution at a more uniform and better distributed rate, and operates without expensive complex material and is capable of continuous operation without servicing and adjustment.
Other objects, advantages and features will become more apparent, as will equivalent devices and methods which are intended to be covered herein, in connection with the teaching of the principles of the invention and the disclosure of the preferred embodiments in the specification, claims, and drawings in which: